1. Field of the Invention
This invention relates to a method and apparatus for increasing the stability of an electrolytic cell by reducing the dissolution of at least one of its electrodes. One important application of the present invention relates to modification of the electrolyte of a molten carbonate fuel cell to reduce the dissolution of a nickel cathode.
2. Description of the Prior Art
In a voltaic cell, chemical oxidation and reduction reactions produce an electromotive force and chemical energy is thereby directly converted to electrical energy. High temperature molten carbonate fuel cells are well known in the art for direct conversion of the chemical energy of hydrocarbons to electrical energy by a galvanic oxidation-reduction process.
Fuel cells, and in particular molten carbonate fuel cells, comprise five principal parts: a fuel chamber to which fuel mixture is fed; a fuel electrode, or anode and its current collector, where gaseous fuel is anodically oxidized by reaction with ions in the electrolyte; an oxidizer chamber to which a mixture of an oxygen containing gas and carbon dioxide is fed; an oxidizer electrode, or cathode and its current collector, where oxygen is galvanically reduced and reacted with carbon dioxide to produce oxygen containing anions; and electrolyte which conducts anions from the cathode to the anode. An external circuit may be provided to conduct the electron stream produced at the anode back to the cathode and thereby produce the desired current flow and electrical balance. To maintain a high level of stable performance, both electrolyte and electrode design and properties must be highly efficient and stabilized at the gas-electrolyte-electrode interface under cell operating conditions. A typical molten carbonate fuel cell is described in "Fuel Cells", edited by George J. Young, Reinhold Publishing Corporation, 1960, pps. 78-93.
Fused carbonates are fundamentally the best adapted salts for high temperature cells, since decomposition by the action of carbon dioxide will not occur and concentration polarization can be eliminated by supplying carbon dioxide withdrawn from the combustion products to the oxidizer chamber. The chemical reactions occurring at the electrodes are as follows: EQU Cathode: 2e.sup.- +1/2O.sub.2 +CO.sub.2 .fwdarw.CO.sub.3 .sup.-- EQU Anode: CO.sub.3.sup.-- +H.sub.2 .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-
Thus, oxygen anions are conveyed through the electrolyte from the cathode to the anode in the form of carbonate anions.
Sophisticated molten carbonate fuel cell electrode configurations, structures and compositions are well known to the art. For example, a preferred molten alkali metal carbonates fuel cell porous anode with a stabilizing agent to maintain high porosity and high surface area is disclosed in U.S. Pat. No. 4,247,604. A porous nickel cathode is preferred for use in a molten carbonate fuel cell with the anode described in U.S. Pat. No. 4,247,604.
Sophisticated electrolyte structures have also been developed for use in molten carbonate fuel cells. Improved molten carbonates fuel cell electrolytes are described in U.S. Pat. Nos. 4,009,321 and 4,079,171 and have an operating composition of about 40 to 70 weight percent carbonates in a high surface area inert alkali metal aluminate support structure, such as lithium aluminate. Under fuel cell operating conditions, at temperatures from about 500.degree. to about 750.degree. C., the entire electrolyte structure, including carbonate electrolyte and inert support material, forms a paste and thus the electrolyte diaphragms of this type are known as paste electrolytes. Porous bubble barriers and composite molten carbonate fuel cell matrices, may be used to provide a gas cross leak barrier to reduce undesired mixing of gases across the electrolyte tile.
U.S. Pat. No. 3,357,861 teaches a diffusion barrier to reduce the problem of diffusion of fuel or oxidant or their products to a counterelectrode through an electrolyte having an ion exchange resin membrane between the electrodes. The diffusion barrier comprises catalysts dispersed in electrolyte or a porous layer comprising a porous plate, sieve, film, or the like, to destroy undesirable fuel or oxidant by chemical action to reduce the gradual buildup of fuel or oxidation product at the electrodes. The use of activated nickel in the diffusion barrier is disclosed.
U.S. Pat. No. 4,404,267 teaches an anode composite for use in a molten carbonate fuel cell comprising a porous sintered metallic anode with a porous bubble pressure barrier integrally sintered to one face of the anode. The porous bubble pressure barrier comprises metal coated ceramic particles sintered together and to the face of the anode by the metallic nickel, copper, or alloys thereof. The pore size of the barrier is significantly smaller than the pore size of the anode. U.S. Pat. No. 4,448,857 teaches a similar cathode composite with a porous sintered bubble pressure barrier of lithium nickel and lithium copper oxides sintered to one face of the cathode.
U.S. Pat. No. 4,411,968 teaches a molten carbonate fuel cell matrix having a matrix tape portion and a bubble barrier portion which rest against the fuel cell anode. Nickel, copper, and alloys thereof, are preferred for the bubble barrier sheet which is prepared by conventional techniques, such as powder sintering.
U.S. Pat. No. 4,137,371 teaches a zinc-oxygen cell with an oxygen electrode with a porous electrically conducting layer, a zinc electrode, and a diffusion barrier of zincate restricting membrane between the porous layer of the oxygen electrode and the zinc electrode. The diffusion restricting membrane is preferably an ion exchange membrane.
U.S. Pat. No. 4,405,416 teaches a molten salt lithium cell having an interface between a lithium electrode and electrolyte to control contact between the electrode surface and the electrolyte by the formation of a protective layer which is believed to be lithium oxide on the electrode.
U.S. Pat. No. 3,772,085 teaches a boundary layer of low halogen content electrolyte adjacent a metallic electrode to prevent electrolyte and halogen from contacting the electrode.